This project involved determination of the physicochemical properties of amino acid residues in dermorphins and deltorphins, environmentally derived from amphibian skin secretions; they exhibit high affinity and selectivity for endogenous mammalian opioid receptors. Applying solid and solution phase peptide chemistry, over 200 synthetic analogues were prepared using natural and unusual amino acids. Receptor studies with membranes from rat brain were statistically analyzed for 1- and 2-site binding models by a stringent iterative mode: fits to 2-site models included Hill coefficient < 0.85, narrow log of the 95% confidence interval ([unreadable] 0.1), P < 0.0001. Importance of the side-chain and chirality of the residues in the N-terminal tetrapeptide region were identified and analyzed: e.g. Aib, Ala, or a series of 1-aminocycloalkane-1-carboxylic acids in positions 2 through 4 revealed that in the heptapeptide, the crucial D-amino acid isomer at position 2 and the critical aromatic side-chain at position 3 could be eliminated by hydrophobic residues. Substitutions of the anionic group by Pro or Abu indicated that the negative charge played a major role in delta receptor affinity, although delta selectivity was dependent on its presence. The acquisition of 5 affinity in some deltorphin analogues, comparable to dermorphin, conferred dual affinity in a single peptide. Based on these culmulative data, we developed a series of di- and tripeptide ultraselective opioidmimetic peptides that represent the universal opioid antagonist message and whose selectivity exceeded that of any known opioid by orders of magnitude. Investigations will continue to develop novel peptide analagues: agonists with potential application for chronic and acute pain, and antagonists to treat narcotic addiction, alcoholism and other clinical syndromes.